tasks created through webapp cannot be accepted/declined in outlook

the designated user receives the task notification in outlook and can now choose to accept or deny it

if the user now accepts it, after confirming to send out an acceptance reply immediately, an error message appears that the change cannot be stored because it has been modified (which is has not been):

when switching to the Outlook task list, the task appears as accepted for the designated user

when looking in the WebApp, the task remains unchanged however (“waiting for an answer”)